The invention herein described was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).
This invention relates to metallic articles having a coating system thereon to permit use at elevated temperatures and, more particularly, to a modified platinum-aluminum coating that may serve as an environmental coating or as the bond coating in a thermal barrier coating system.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys and can operate at temperatures of up to 1900-2100xc2x0 F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cool air is forced through the channels during engine operation.
In another approach, a protective environmental coating or a ceramic/metal thermal barrier coating (TBC) system is applied to the turbine blade or turbine vane component, which acts as a substrate. The protective environmental coating is useful in intermediate-temperature applications. One known type of metallic protective coating is a platinum-aluminum coating that is formed by depositing platinum and aluminum onto the surface of the substrate, and interdiffusing these constituents.
A ceramic thermal barrier coating may be applied overlying the platinum-aluminum coating to form a thermal barrier coating system. The thermal barrier coating system is useful in higher temperature applications. The ceramic thermal barrier coating insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. However, ceramic layers usually do not adhere well directly to the nickel-base superalloys used in the substrates. To improve ceramic coating adhesion and provide oxidation resistance to the substrate in case of ceramic spallation, a bond coating is applied to the substrate. (Bond coats are also sometimes termed diffusion aluminides or overlay coatings.) Platinum aluminides, the focus of the present invention, are one example of a diffusion aluminide. The platinum-aluminum bond coat is placed between the substrate and the ceramic thermal barrier coating to effect the adhesion of the ceramic layer to the substrate. In addition, the upper surface of the bond coat oxidizes to a protective aluminum oxide scale to inhibit further oxidation of the substrate.
Although superalloys coated with such environmental coatings and ceramic/metal thermal barrier coating systems do provide substantially improved performance over uncoated materials, there remains an opportunity for improvement in elevated temperature performance and environmental resistance. There is an ongoing need for improved environmental coatings and bond coats to protect nickel-base superalloys in elevated-temperature applications. This need has become more acute with the development of the newest generation of nickel-base superalloys, inasmuch as the older protective coatings are often not satisfactory with these materials and higher temperature performance requirements. The present invention fulfills this need, and further provides related advantages.
The present invention provides a articles with an environmental coating or a thermal barrier coating (TBC) system and methods for their preparation. The articles have improved service life at elevated temperature as a result of reduced degradation through spallation and consequent loss of the coating. The functionality of the thermal barrier coating system to insulate and protect the substrate is retained for longer exposure to elevated temperature and greater numbers of thermal cycles.
In accordance with the invention, an article comprises a substrate, and coating system deposited on the substrate. The thermal barrier coating system includes a protective layer overlying the substrate and, optionally, a ceramic thermal barrier coating layer overlying the protective layer. The protective layer has a composition comprising platinum and aluminum, plus, in atomic percent, from about 0.14 to about 2.8 percent hafnium and from about 2.7 to about 7.0 percent silicon. The atomic ratio of silicon:hafnium is desirably from about 1.7:1 to about 5.6:1. The hafnium and silicon compositions are average values measured through an additive layer, as will be described subsequently. The aluminum content of the protective layer is any operable amount, preferably from about 30 to about 60 atomic percent, most preferably from about 30 to about 50 atomic percent. The platinum content is any operable amount, preferably from about 4 to about 15 atomic percent, most preferably from about 4 to about 13 atomic percent. The balance of the protective layer is elements diffused into the layer from the substrate, such as nickel, cobalt, chromium, tungsten, etc.
A preferred method to prepare the article includes the steps of providing a substrate, depositing layers containing the platinum, aluminum, hafnium, and silicon, and heating the layers so that the aluminum, hafnium, and silicon diffuse into the layer of platinum to form a protective layer. An additive layer portion of the protective layer has an average composition as set forth above. If a thermal barrier coating system is desired, a ceramic thermal barrier coating layer is deposited overlying the protective layer, either after the diffusing is complete or simultaneously with the interdiffusing step.
The coating system of the invention exhibits increased service life as a result of improved resistance to degradation by spallation. When the platinum aluminide layer is exposed to elevated temperatures, its surface oxidizes to form an aluminum oxide layer. The principal failure mode is the spalling of the aluminum oxide layer as a result of the formation of cracks in the aluminum oxide or at the aluminum oxide/platinum aluminide interface. The presence of the indicated additions of silicon and hafnium inhibits this failure mode, reducing the incidence of cracking in the aluminum oxide and at the aluminum oxide/platinum aluminide interface. The coating system modified with the addition of silicon and hafnium in the indicated amounts remains functional for a longer period in service conditions than does the unmodified protective layer.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.